Weather forecasts and discussions for Canada

Where are my tree leaves?

Living in Eastern, Central and Northern Ontario you might be wondering why it seems to take so long for the trees to leave out? Take a trip down to Hamilton, Niagara, London, Windsor or even the GTA you might notice a world of difference, as the trees have been blooming for a while in that region.

Well it’s true that weather plays a pretty big part in waking trees up from their winter slumber. The trees react to a series of warm days to signal them its time to start blooming. However most species of trees in the Northeast region of North America take it pretty slow, as they know a flip to freezing temperatures can occur destroying the tender blooms. However, there are even more interesting reasons when a tree knows to start the blooming process.

In terms of weather, one of the reasons this year bloom is a little slower in some parts of the province has been a lack of those warm day after day temperatures. For example, Ottawa was around 1 degree above normal for the daily high temperatures in April. Ottawa only seen 2 days above 20c, rather largely spread apart from each other with only a few warm days in a row since April 27th. In comparison, London, Ontario was +3 degrees above normal for the month with over 7 days above 20c, many of the warmth spread over consecutive days. This increased temperature would play a big factor in getting trees and plants growing. Although the above two location are geographically far apart with London quite a bit further south, the two cities are less than 1 degree apart for the typical average daily high in April. The normal high temperature for Ottawa in April in 11.6 Celsius while London’s average is 12.1 Celsius.

However interestingly enough, weather and warmth is not the number one factor for trees to bloom; otherwise they would have began to bud during our winter thaws. Increased daylight is the number one factor for plants and trees to know when to bloom. However, since Ottawa and London see roughly the same amount of hours of daylight in April (around 13 hours per day), I would say the increased temperature in Southern Ontario is what’s bumping their trees ahead.

The other factors effecting bloom are water (humidity) and nutrition. Since trees produce the buds for the following spring during the past summer, the fact that Eastern Ontario, and many other regions in Ontario had a very hot and dry summer that carried through to the fall could effect the number of buds that will be available this spring.

The most interesting factor for trees that also has to do with the weather is the winter! Most species of trees need to have a certain number of days with freezing temperatures (below -5c) before they bloom in the spring. This sets the trees “internal clock” to know when to bloom, meaning they need a number of hours throughout the winter with those cold temperatures. This is the reason that trees know not to bud out in January or February during our warm thaws. The hours vary by species of tree and native location however its generally around 2,000 hours in Northeastern North America. Certain species such as Aspen and Poplar need less are among the first to break bud, while some need more like Ash and Oak which are later in the spring. Also the trees native location plays a part, with species from Florida only needing a few weeks of colder temperature, while northern species require months. This is why you have to pay attention to the gardening hearty zones when planting trees.

In reality Eastern, Central and Northern Ontario trees are near normal for their bloom time and only slightly behind; they have been even later in colder springs. In fact, too early can be a disaster as we seen in March 2012 when temperatures soared to near 30c for almost a week to only fall well below normal by mid April. This killed off many of the fruit trees blooms and affected the growing and fruit season later that year. It’s always best when mother nature plays it safe and we see later blooms.